def keyval_not_none(self, key: str, value: Optional[T],
transform: Optional[Callable[[T], Any]] = None) -> 'JsonWriter':
""" Write out an object element only if the value is not None.
def keyval_not_none(self, key: str, value: Optional[T],
transform: Optional[Callable[[T], Any]] = None) -> 'JsonWriter':
""" Write out an object element only if the value is not None.